In order to study the molecular mechanisms involved in signal transduction and regulation of catalytic activity of the putative growth factor receptor gene, c-erbB-2, a series of mutants in different structural domains of the mature gene product have been generated by means of site-directed mutagenesis techniques. Mutant molecular clones were then inserted into eukaryotic expression vectors and expressed in NIH/3T3 cells in order to assess the biologic activity in a focus assay. A nonconservative amino acid substitution in the transmembrane domain leading to a change from valine to either aspartic or glutamic acid activates the transforming potential of the gene. This finding suggests that the transmembrane domain is important in signal transduction and that specific molecular lesions might irreversibly mimic informational changes which usually take place reversibly upon ligand binding to the receptor. Further work is aimed at correlating biological differences between these mutants and the wild-type molecule in a variety of biochemical assays. Experiments are also in progress to evaluate the biologic activity of another series of mutants generated in the COOH terminal region of the protein.